1. Field of the Invention
The present invention relates to an electronic circuit device and, more specifically, to a package-type hybrid IC molded with resin.
2. Description of the Related Art
Conventional resin-packaged hybrid ICs can be divided into those of the SIP type and those of the DIP type depending upon their structure for providing leads out of the package. In the case of hybrid ICs molded with resin, the circuit board is inserted in advance in a metal mold or in a case and is sealed with a liquid or powdery thermosetting resin (such as an epoxy resin or a silicone resin) using a casting method, a dip method, a transfer method or a similar method.
However, the above-mentioned conventional hybrid ICs develop stress during the step in which the molding resin is heated and cured or during the step in which the resin contracts due to cooling. Moreover, residual stress develops between the molding resin and the circuit board due to differences in the coefficient of contraction and the coefficient of thermal expansion therebetween. These stresses act on the resistors which are secured to the circuit board and, as a result, the resistances of the resistors undergo a change due to stresses exerted thereon.
According to the test and analysis conducted by the present inventors, it was discovered that the compression stress and bending stress in a direction perpendicular to the surface act upon the circuit board due to the contraction of the molded resin portion, and the resistors are subjected to compression or tensile stress due to the bending stress. The bending stress varies depending upon the portions on the circuit board, whereby the resistance of a resistor may vary depending upon the position where it is formed on the circuit board and, hence, the circuit employing these resistors produces varying outputs.
The inventors have further confirmed the fact that the resistance of the resistors varies due to the stress exerted from outside the hybrid IC.
FIG. 32 illustrates a hybrid IC of the conventional SIP type. A circuit board 2a is accommodated in a case 1a, and circuit elements such as resistors R1, R2 and the like are formed on the circuit board 2a. Reference numeral 31a denotes a molded resin portion which is filled in the case 1a to prevent the circuit elements from getting wet and to fasten the circuit board. Reference numeral 9a denotes a protection glass that protects the resistors R1 and R2.
According to another example of the hybrid IC of the SIP type, a Silicon-Gel film is thinly applied to the whole surface of the circuit board 2a which is then accommodated in the case 1a followed by filling with the molding resin 3a.
According to the above-mentioned conventional resin-packaged hybrid IC, however, stress builds up between the sealing resin portion 3a and the circuit board 2a due to differences in the coefficient of contraction and the coefficient of thermal expansion therebetween during the step in which the sealing resin contracts upon curing or during the step in which the resin expands or contracts due to the subsequent heating or cooling. The stress acts upon the resistors R1 and R2 secured to the circuit board 2a and, hence, the resistances of the resistors R1 and R2 undergo a change due to the stress acting thereupon.
In the conventional hybrid IC constitution shown in FIG. 32, in particular, the elements are mounted on one main surface of the board 2a. Therefore, the board 2a is not mounted at a central position in the case 1a but is inevitably positioned so as to be deviated toward one side of the case.
When the board 2a is disposed so as to be deviated in the case 1a, a difference develops in the stress that is generated by the sealing resin 3a between the surface of the board on which the elements are mounted and the surface of the board on which no elements are mounted and, in addition, the sealing resin 3a is filled in different amounts in the space that is formed between one surface of the board 2a and the case 1a and in the space that is formed between the other surface of the board 2a and the case 1a. Due to these differences, therefore, different stresses build up on the two surfaces of the board 2a.
The present invention was accomplished in view of the above-mentioned problem, and its first object is to provide an electronic circuit device which is capable of decreasing dispersion in the output of the circuit that is caused by changes in the resistances of the resistors as a result of stress of hardening and contraction taking place between the molded resin portion and the circuit board, stress due to the difference in the coefficient of thermal expansion, and external stress.
A second object of the present invention is to provide an electronic circuit device which is capable of decreasing or eliminating changes in the resistances of the resistors caused by stress that builds up between the molded resin portion and the circuit board.